Kamel is right, though Visual C# 2008 Express Edition has less features than other editions of Visual Studio, as it is free, but nevertheless you are able to compile D3's sources using this IDE. I have specially downloaded and installed it and checked this.
You can simply ignore these warnings.
DynamicDataDisplay.Maps contains custom build action for building pixel shader that implements mercator transform for tiles of maps. There is no security risk in this project, though IDE reports about this.
Also, you should decide, what platform you are interested in - WPF or Silverlight, as the first is a desktop applications technology, and the second is for web applications.
I have not tested if Silverlight version of our project can be compiled in Visual C# 2008 Express Edition.
.NET itself also contains everything that is necessary to build .NET projects - there is a msbuild and C# console compiler, of which I'm not well informed but which which are capable of compiling everything that Visual Studio can build.
Also you can give a try to other freeware C# IDEs - for example, SharpDevelop - http://www.icsharpcode.net/OpenSource/SD/. I have not tested it in building D3's sources, too, but maybe it will